1. Field of the Invention
The present invention relates to a positive resist composition and a method of forming a resist pattern. Priority is claimed on Japanese Patent Application No. 2003-169833, filed Jun. 13, 2003, the content of which is incorporated herein by reference.
2. Background Art
In recent years, in the production of semiconductor elements and liquid crystal display elements, advances in lithography techniques have lead to rapid progress in the miniaturization of semiconductor elements and liquid crystal display elements. Typically, these techniques for achieving further miniaturization involve shortening of the wavelength of the exposure light source. Until recently, ultraviolet radiation such as g-lines and i-lines have been used as the exposure light source, but recently, mass production using KrF excimer lasers (248 nm) has been started, and even ArF excimer lasers (193 nm) are now starting to be introduced. Radiation of even shorter wavelengths such as F2 excimer lasers (157 nm), EUV (extreme ultraviolet radiation), electron beams, X-rays and soft X-rays is also being investigated.
Furthermore, with the rate of miniaturization development growing ever faster, levels of resolution capable of forming line and space patterns of no more than 100 nm, hole patterns of no more than 140 nm, and isolated patterns of no more than 70 nm are now being sought.
As a result, research and development is also focusing on the method of forming the resist pattern, in an effort to exceed the current resolution limits imposed by the resist material.
An example of one such miniaturization technique that has recently been proposed (for examples, see Japanese Unexamined Patent Application, First Publication No. 2000-356850, Japanese Unexamined Patent Application, First Publication No. 2000-188250) is the thermal flow process, wherein a resist pattern is formed using normal lithography techniques, and the resist pattern is then subjected to heat treatment to reduce the pattern size. Thermal flow is a method in which following formation of a resist pattern using photolithography techniques, the resist pattern is heated and softened, causing the pattern to flow towards the gaps in the pattern, thus reducing the size of the portions where the resist pattern is not formed (such as the hole diameter in a hole pattern, or the space width in a line and space (L&S) pattern).
On the other hand, one example of a known resist material that satisfies the high resolution requirements needed to enable reproduction of a pattern with very minute dimensions is a chemical amplification type resist composition comprising a base resin that can change alkali solubility thereof under the action of acid, and an acid generator that generates acid on exposure to light, dissolved in an organic solvent.
In KrF excimer laser lithography, positive resist compositions and the like comprising, as the base resin, a polyhydroxystyrene that has high transparency relative to KrF excimer laser radiation (248 nm) or a polyhydroxystyrene based resin in which a portion of the hydroxyl groups have been protected with an acid dissociable, dissolution inhibiting group are typically used as chemical amplification type resist compositions (for examples, see Japanese Unexamined Patent Application, First Publication No. Hei 4-211258, Japanese Unexamined Patent Application, First Publication No. 2002-287363). Examples of the most commonly used acid dissociable, dissolution inhibiting groups include so-called acetal groups, including chain-like ether groups such as 1-ethoxyethyl groups and cyclic ether groups such as tetrahydropyranyl groups, as well as tertiary alkyl groups such as tert-butyl groups, and tertiary alkbxycarbonyl groups such as tert-butoxycarbonyl groups.
In this type of positive resist composition comprising a polyhydroxystyrene based resin as the base resin, in order to achieve a high resolution resist pattern, the proportion of hydroxyl groups within the resin that are protected by acid dissociable, dissolution inhibiting groups is preferably high.
However, as the proportion of hydroxyl groups protected by acid dissociable, dissolution inhibiting groups is increased, the dissolution inhibiting effect strengthens. As a result, a problem arises in that the exposed portions do not dissolve satisfactorily in the developing liquid, and a residual scum is generated within the exposed portions. Furthermore, the occurrence of developing defects also becomes more prevalent.
Furthermore, this resin has substrate dependency, with scum generation being particularly marked in those cases where a nitrogen-containing substrate such as titanium nitride is used. In addition, depending on the property of the acid dissociable, dissolution inhibiting group, the level of hydrophobicity within the unexposed portions can increase, resulting in a deterioration in the compatibility with the rinse liquid used in the rinse step following developing, and causing residues to be left adhered to the unexposed portions.
Accordingly, there is a limit to how far the proportion of hydroxyl groups protected by acid dissociable, dissolution inhibiting groups can be increased, and obtaining a high resolution resist pattern, while suppressing the level of scum and developing defects has been difficult.
Furthermore, Japanese Unexamined Patent Application, First Publication No. 2002-287363 discloses a chemical amplification type positive resist composition with improved levels of scum suppression, in which polypropylene glycol is added to the base resin comprising a polymer of hydroxystyrene and 2-ethyl-2-adamantyl (meth)acrylate. However, although a positive resist using this polymer displays improved scum suppression, the resolution and depth of focus are not entirely satisfactory.